The biosynthesis (in situ, in tissue preparations and in cell-free systems) of proparathyroid hormone and its conversion to parathyroid hormone in mammalian parathyroid glands is under investigation. Emphasis is placed on the chemical structure of the prohormone, other peptides which may be related to parathryoid hormone in a potential precursor role, the enzymes responsible for the conversion of the prohormone to parathyroid hormone, the packaging and secretion of parathyroid hormone and the control of these processes by calcium and other ions. In vitro studies are underway on tumor tissue from patients suffering hypercalcemia related to malignancy to determine if this type of tissue synthesizes and secretes parathryoid hormone or a parathyroid hormone-like peptide.